Polycrystalline silicon is used as a material of a monocrystalline silicon substrate for semiconductor device manufacturing and a substrate for solar cell manufacturing. As a method of producing the polycrystalline silicon, the Siemens method is known. The Siemens method is a method of bringing a source gas containing chlorosilane into contact with a heated silicon core wire and thereby vapor-phase growing the polycrystalline silicon on the surface of the silicon core wire with a CVD method to obtain the polycrystalline silicon as a silicon rod.
When the polycrystalline silicon is vapor-phase grown by the Siemens method, two silicon core wires in the vertical direction and one silicon core wire in the horizontal direction are assembled in a square arch shape (U-shape) in a reaction furnace of a vapor-phase growing apparatus. Both the ends of the silicon core wires of the square arch shape (U-shape) are fixed to a pair of metal electrodes, which are disposed on a reaction furnace bottom plate, via a pair of core wire holders. A supply port of a source gas for causing reaction and an exhaust port of a reaction exhaust gas are also disposed on the bottom plate. Such a configuration is disclosed in, for example, Japanese Patent Laid-Open No. 2011-231005 (Patent Literature 1).
In general, in a reaction furnace, several tens of the silicon core wires of the square arch shape (U-shape) fixed to the pair of metal electrodes disposed on the bottom plate are provided and are disposed in a multiple annular type. In recent years, according to an increase in demands for the polycrystalline silicon, an increase in the size of a reaction furnace for increasing production has been advanced. A method of depositing a large amount of the polycrystalline silicon in one batch is adopted. According to this tendency, the number of silicon core wires disposed in the reaction furnace also increases.
However, when the number of silicon core wires set in the reaction furnace increases, a short supply of chlorosilane to respective polycrystalline silicon rod surfaces occurs. Such supply instability of the source gas causes unevenness (popcorn) on the surface of the silicon rod. As a result, the thickness of the silicon rod becomes non-uniform and a shape failure occurs. When the unevenness occurs on the silicon rod surface, the polycrystalline silicon tends to abnormally grow. Further, a cleaning effect in cleaning before shipment of the polycrystalline silicon is greatly deteriorated. To eliminate the unevenness of the silicon rod surface, the temperature (reaction temperature) of the surface of the silicon rod only has to be lowered to moderate a deposition reaction. However, in this case, deposition speed of the polycrystalline silicon decreases and productivity and energy efficiency are markedly deteriorated.
Under such circumstances, as a method of suppressing the occurrence of the popcorn and improving the deposition speed for productivity improvement, various methods have been proposed as a method for efficiently supplying the source gas to the silicon rod surface. For example, in methods disclosed in Japanese Patent Laid-Open No. 2011-231005 (Patent Literature 1) and Japanese Patent Laid-Open No. 2003-128492 (Patent Literature 2), a source gas amount to be supplied to the silicon rod surface is adjusted to efficiently promote the deposition reaction by adjusting a source gas supply nozzle shape and a flow rate of the source gas to be supplied.
All of these prior patent literatures are techniques for improving productivity, that is, keeping the deposition speed of the polycrystalline silicon in a high state and reducing the popcorn by adjusting a reaction temperature and source gas concentration (a supply source gas amount) near the silicon rod surface in the reaction furnace.
On the other hand, the inventors propose a technique for preventing stagnation of the source gas in the reaction furnace and suppressing occurrence of popcorn and occurrence of silicon powder (Patent Literature 3: WO2012/098598).
In this technique, when the area of a bottom plate is represented as S0, using a reaction furnace for polycrystalline silicon producing designed to provide all material supply nozzles on the inside of an imaginary concentric circle having the center in the center portion of the bottom plate and having an area S=S0/2, a source gas is supplied from ejection ports of the source gas supply nozzles at flow velocity equal to or higher than 150 m/sec to form an overall reflux of the source gas in the reaction furnace such that a flow pattern of the reaction gas is an ascending current in a reaction furnace center portion and is a descending current in a reaction furnace outer wall side portion.